Oak wilt, or decline, is an infectious disease caused by the fungus Ascomycete Ceratocystis Fagacearum. Its pathogenicity arises because it enters and clogs the vascular system of the tree's roots, trunk and limbs so that it cannot carry the water and nutrients necessary to survival of the tree. During the summer when the disease is most virulent, a live oak may lose all its leaves within thirty days of infection. Most will die within six months to two years. The more susceptible Spanish oaks often die seven to thirty days after infection.
At present, oak wilt disease has reached epidemic proportions in many counties in Texas. It is also present in trees in Minnesota, Pennsylvania and South Carolina. There is every reason to think that the disease will continue to spread, and will eventually infect trees in every state and in other countries as well.
One of the first symptoms of oak wilt is veinal necrosis of the leaves--the leaf dies and turns brown along the veins but remains alive and green elsewhere. Later, the leaves show greater degrees of necrosis. Defoliation usually follows quickly. Trees that are not killed outright by the disease may be weakened to the point where they are susceptible to other pathogens. Some trees may remain in a defoliated state for months or even years.
The fungus quite often spreads locally from tree to tree through rootgrafts and common root systems, i.e., where roots of different trees merge and grow together. It can also be spread through human activity, such as through the use on healthy trees of infected tree pruning equipment. When an infected tree is pruned, fungal spores can remain on and be carried by pruning equipment to a healthy tree. The spores are effectively "injected" directly into the healthy tree's vascular system when pruning cuts are made in its limbs.
Insects can spread the fungus over long distances. Infection of distant trees by insect vectors is particularly likely in trees where fungal mats are formed. Fungal mats are conglomerations of fungus which are typically observed in the Texas red oak. They usually form under cool, moist conditions. The conglomeration acts to push bark away from the tree wood leaving access for insects to the interior of the tree. The sweet odor of the fungal mat actually attracts insects to the interior of the tree where they contact the fungus. Insects then will often carry fungal spores on their bodies to wounds and pruning cuts of healthy trees, thereby infecting them.
It is also likely that birds, rodents, and squirrels carry spores from infected to uninfected trees. Woodpeckers, which bore into the tree, and insect borers such as the Texas Longhorn Borer, are also believed to be culprits, since they come into contact with the vascular systems of both infected and uninfected trees.
Firewood also plays an important role in the spread of the fungus, in that it may harbor insects which spread spores. Insects often spend the winter in piles of firewood and if the wood is infected, will emerge in the spring carrying oak wilt spores. The effect of firewood is particularly noteworthy because it is often transported great distances by humans, thereby leading to the spread of the disease to new areas.
Although several methods have been proposed in the past for combating the spread of the disease, the experts agree that there is no known cure for oak wilt. See Texas Weekly Magazine pp. 6-9 (May 25, 1986) (quoting Dr. D. Appel); R. F. Billings & R. S. Cameron, "Texas Forest Service Forest Pest Suppression Project: Oak Wilt," pp. 27-32 (September 1987) (proposing a method for controlling the spread of the disease).
One method in particular has been recommended for controlling tree-to-tree infection. A trench two to three feet deep, or as deep as the majority of the lateral roots, is dug around the infected trees. Where there is a group of infected trees, a trench is dug around the entire group. The trench is designed to create a barrier to prevent root-to-root infection. As a further precaution it has been recommended that infected trees within the trenched area be uprooted, and that any Texas red oaks--in which fungal mats can form--be burned immediately. As an alternative to trenching, severing the roots chemically with the soil fumigant Vapam has been proposed.
Severing root systems with either method, of course, does nothing to prevent new infections by vectors such as insects, rodents, birds, or squirrels. Moreover, Vapam has not proved effective in field trials conducted in Texas. See R. F. Billings & R. S. Cameron, "Texas Forest Service Forest Pest Suppression Project: Oak Wilt," p. 31 (September 1987).
A more drastic method of control is to kill infected trees with silvicides. This method is less desirable, however, because it not only kills the "patient," but also can kill healthy trees in the area where the silvicides are applied. Silvicides can be absorbed through the root systems.
Any of these prior art methods, coupled with other common sense measures such as thoroughly cleaning pruning equipment, avoiding transport of infected firewood, and avoiding transplanting infected trees, can be somewhat effective in controlling, or at least slowing, the spread of the disease. However, all of these prior art control methods suffer from two serious disadvantages--none can entirely stop the spread of the disease, and none offers a cure for infected trees. It is clear that a more effective treatment is needed to halt the spread of this menace to the great oak trees.